Impurities in the form of solid particles contained in a liquid (for example, ink in recycled-paper pulp suspension) can be removed by injecting gas (usually air) into the liquid to form bubbles, permitting the gas bubbles (to which the impurities attach on their surfaces) to rise to the surface of the liquid so as to form a foam, and removing the foam floating on the surface of the liquid. For example, Hellberg U.S. Pat. No. 4,186,094 describes deinking apparatus employing an air injector in which the pulp suspension is forced to flow through a passage between concentric cones or between flat plates, and air is injected into the suspension through pores in a cone or a plate.